The present invention relates generally to methods and apparatuses for monitoring optical signals, and more particularly to the monitoring of optical signals with an improved frequency resolution.
In a typical fiber optic transmission network such as, for example, a wavelength division multiplexed (WDM) network information is transported between optical terminals by optical fiber links characterized with optical channels operating at distinct wavelengths. The use of optical fibers to carry information substantially increases the distance separating optical network terminals. However, standard optical terminal interconnections are nevertheless limited by a number of factors including the optical power that can be launched into the interconnecting fibers, fiber loss, fiber dispersion and the sensitivity of optical receivers used in the optical terminals.
Where the distance between desired end points of an optical fiber transmission network exceeds the maximum distance between optical terminals over which information can be reliably transmitted, transit terminals such as optoelectronic repeaters and optical amplifiers are commonly used along the transmission path for signal amplification and regeneration. Typically, these transit terminals are placed between fiber spans that can each extend from 40 to 100 km.
In most systems, the performance of this transit equipment must be monitored so that faults in operation of the optical transmission network can be isolated. This monitoring, typically referred to as link quality monitoring (LQM), helps determining whether a particular network is within specified performance requirements.
In order to effectively LQM an optical transmission network, it is well known to monitor and administrate the transmission of information in each fiber on a per-channel basis by using a small portion of the available channel bandwidth in the fiber to transmit a low frequency, low amplitude dither signal. According to this method, each channel signal is modulated before transmission with an amplitude dither signal. At a transit terminal receiver, the embedded dither signal is recovered by correlation with known dithering sequences.
As is well known, the recovered dither signal in each channel can assist in monitoring various transmission parameters downstream from the point of transmission. For example, the dither can provide a downstream estimation of the optical signal power received from a particular optical channel, assist in determining the optical signal-to-noise ratio (SNR) of a particular channel signal or provide an indication as to whether a channel signal is present in a particular channel.
Presently, in order to retrieve the dither from an optical channel signal, the optical channel signal (or a portion thereof) must first be demodulated and converted into an electric form before any manipulation of the dither information can be carried out. Unfortunately however, this would dramatically increase the cost of monitoring data transmission. As optical technology evolves toward all optical networking, there will be a need to monitor optical channel signals optically at various points without having to do a complete optical channel demodulation.
However, a high modulation depth for the dither reduces the signal-to-noise ratio (SNR) of the channel signal and results in a substantial degradation of the system""s performance. Without a higher modulation depth, transmission monitoring over distances extending beyond 7 spans cannot be carried out with sufficient accuracy. With a higher modulation depth, the transmission of information over these long distances can be more effectively monitored but channel signal SNRs may as a result be detrimentally affected causing a significant degradation in performance. As the distance between desired end points in an optical fiber transmission network can generally exceed 7 spans, it would be desirable to increase the distances over which data transmissions can be reliably monitored without causing any penalty in performance.
These limitations have led to the development of all-optical monitoring techniques which are typically embodied into optical spectrum analyzers. With conventional optical spectrum analyzers, the transmission of information in a fiber can also be monitored on a per-channel basis. Typically, the optical channel signal is travelling in the fiber are optically separated and their respective optical intensity is measured to determine the amount of light propagating in each channel. By measuring the optical intensity of each channel signal, several channel transmission parameters can be monitored including the optical channel signal power, the channel SNR and the amplified spontaneous emissions (ASE) present in each channel. Contrary to dithering techniques, these channel parameters can be measured without requiring any signal demodulation and without relying on a pre-knowledge of dither information placed on each channel signal.
There are many types of optical spectrum analyzers which are known in the art. Conventional optical spectrum analyzers typically provide a wide spectrum range and often include a dispersive element such as a Bragg cell with an array of photo-detectors. A tunable dispersive element with a single photo-detector is also commonly used. With these key components, conventional optical spectrum analyzers can monitor many channel signals over a relatively large bandwidth by tuning the dispersive element to repeatedly sweep along the optical spectrum and measure the optical light as a function of wavelength.
However, the frequency resolution conventionally achieved is quite low. Apart from the large size and low sweeping speed which is limited by the mechanical tuning of the dispersive element, the maximum resolution bandwidth of known spectrum analyzers is typically in the order of 10 GHz. Unfortunately, this resolution is not adequate to monitor closely spaced channels or distinguish between different types of optical traffic.
Various attempts have been made to improve the existing resolution. However, most of these attempts have been very expensive or have created other problems in the signal processing operation. For example, in the past, it has been attempted to improve the frequency resolution of the system by increasing the length of the Bragg cell in order to extend the frequency resolution. However, this approach is not especially productive inasmuch as the extended length of the Bragg cell has the inherent effect of substantially attenuating the optical fiber signal which is supplied thereto whereby distortion of the optical output frequency signal is produced. Also, in many systems, such elongated Bragg cells become very expensive, delicate to handle, and hard to package.
The present invention addresses these issues and to this end provides a methodology and apparatus to mitigate the present limitations in this art.
The present invention provides a method and apparatus for monitoring optical signals with an expanded frequency resolution. The invention permits high-resolution measurement of optical signal spectrums while retaining wide bandwidth operation through appropriate electronic control circuitry.
In order to achieve high resolution in monitoring a particular optical signal, the invention uses an interferometer to sweep the entire signal spectrum. As is well known, the interferometer has a periodic transfer function which consists of equally spaced narrow-band peaks each tunable to a particular narrow-band wavelength range. According to the invention, the interferometer frequency response is incrementally tuned in cycles so that each of its frequency response peaks cyclically scans a particular spectral band. During each cycle, the interferometer isolates multiple spectrally resolved portions of the optical signal spectrum where each portion originates from a different spectral band. By operating the interferometer to scan each band completely, a high-resolution measurement of the entire signal spectrum can be obtained.
In order to adequately process the spectrally resolved portions and obtain a complete spectral measurement of the incoming optical signal, the spectrally resolved portions are separated in the space domain as a function of wavelength. According to the invention, different methods can be used to space domain separate the spectrally resolved portions of the signal spectrum. For example, this can be accomplished by first separating the incoming optical signal spectrum into the different wavelength regions or bands to be scanned with an array of optical filters and then sequentially applying the bands separated to a scanning interferometer. Alternatively, instead of separating the incoming signal spectrum into bands before any interferometry is applied, interferometry could be applied to the incoming signal first to produce spectrally resolved portions of the signal spectrum and then separating the portions obtained in the space domain with the array optical filters.
According to the invention, the spectrally resolved portions can also be separated in the time domain as a function of time. For example, this can be accomplished by coupling the optical signal to wavelength-dependent delay lines for time-delaying the signal spectrum in bands as a function of wavelength and sequentially passing the time-delayed bands into the interferometer to produce the spectrally resolved portions in sequence.
The invention can be advantageously incorporated into an optical spectrum analyzer or directly into any optical terminal and used for link quality monitoring (LQM) in optical communications networks.
In contrast to conventional dithering methods used for LQM, the invention advantageously enables the optical monitoring of an extended range of transmission parameters including carrier wavelengths, optical SNRs, ASE noise levels, optical non-linearities or other signal baseband information such as data rates and formats.
Another advantage of the invention over dithering as a means for monitoring transmission is that the invention is protocol and vendor independent which substantially reduces the complexity of the LQM signal processing required in each optical terminal.
Another advantage of the invention over dithering is that data transmissions can be reliably monitored over greater distances.
The invention also advantageously provides a much higher resolution than that provided by conventional optical spectrum analyzers. With this higher resolution, closely spaced channels and optical streams operating at different speeds can be more comprehensively monitored.
Another advantage of the invention over conventional optical spectrum analyzers is that the use of a scanning interferometer substantially improves the speed of acquisition.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.